(1) Field of the Invention
The present invention generally relates to a method for fabricating an exposure mask, such as an X-ray exposure mask, which is used for manufacturing semiconductor devices; and more particularly to a method for fabricating an exposure mask which includes a step of adhering a substrate to a supporting block.
(2) Description of Related Art
FIG. 1A is an end view showing an X-ray exposure mask, FIG. 1B is a top view showing the X-ray exposure mask, and FIGS. 2A-2E illustrate a conventional method for fabricating the X-ray exposure mask.
Referring to FIGS. 1A and 1B, an X-ray mask 1 has a reinforcement block 2 (a supporting block), a silicon ring 3, a membrane 4 that is transparent to X-rays, and an X-ray absorbing pattern 5. The reinforcement block 2 serves for mechanically reinforcing the X-ray mask 1. The reinforcement block 2 may be made of SiC. The reinforcement block 2 is ring-shaped. The silicon ring 3 is adhered to an upper main surface of the reinforcement block 2 by an adhesive 6. The membrane 4 which is made, for example, of SiC, is provided on the silicon ring 3. The X-ray absorbing pattern 5 which is made, for example, of Ta, is formed on the membrane 4. The X-ray pattern 5 is patterned according to the desired semiconductor pattern as usual.
The X-ray mask 1 having the above structure is formed in accordance with a process shown in FIGS. 2A-2E.
In the step of FIG. 2A, the membrane 4 is deposited on an upper major surface of a silicon wafer 7 by a CVD process. The membrane 4 is made, for example, of Sic as already noted. Then an X-ray absorbing film 8 which is made, for example, of Ta is deposited on the membrane 4 by sputtering.
In the step of FIG. 2B, the adhesive 6 which is a sheet-shaped thermosetting adhesive is provided on the reinforcement block 2, and a layered body 107 shown in FIG. 2A is placed on the adhesive 6 so that the adhesive 6 is sandwiched between the reinforcement ring block 2 and the silicon wafer 7.
In the step of FIG. 2C, a structure formed of the reinforcement block 2, the adhesive 6, the silicon wafer 7, the membrane 4, and the X-ray absorbing film 8, shown in FIG. 2B, is set in a concave part 11 of a heater plate 10 which contains a heater 9 therein. Then, a pressure device 12 is brought into contact with a fringe area on the upper major surface of the X-ray absorbing film 8 directly or via sheet of soft material such that the pressure device 12 presses the stacked structure against the heater plate 10 for a predetermined time (e.g. 5 min.). That is, the silicon wafer 7 is pressed via the adhesive 6 to the upper major surface of the reinforcement block 2. During this process, electric power is supplied to the heater 9 so that the adhesive 6 is heated at its curing temperature (e.g. 180.degree. C.). As a result, the silicon wafer 7 is firmly adhered to the upper major surface of the reinforcement block 2 by the adhesive 6.
In the step of FIG. 2D, an etching process is carried out on a lower surface of the silicon wafer 7 until a lower surface of the membrane 4 is exposed, so that the silicon ring 3 is formed from the silicon wafer 7.
In the step of FIG. 2E, an etching process is carried out on the X-ray absorbing film 8 so that the X-ray absorbing pattern 5 is formed on the membrane 4.
In accordance with the above process, the X-ray exposure mask shown in FIGS. 1A and 1B is formed. The above X-ray absorbing pattern 5 can be also formed on the membrane 4 before the silicon wafer 7 is adhered to the reinforcement block 2.
In the above conventional method for fabricating the X-ray exposure mask, the pressure device 12 is employed in direct contact with the layered body 107 including the silicon wafer 7 to press the layered body 107 against the reinforcement block 2 when the layered body 107 is adhered to the reinforcement block 2 (the supporting block) by the adhesive 6. Thereby, there is a risk that unwanted dust particles present on the pressure device 12 may be transferred to a surface of the X-ray absorbing film 8. When such dust particles exist on the surface of the X-ray absorbing film 8, there is a substantial risk that the mask pattern may be damaged upon the proximity exposure process that is conducted by using the mask. Once present, it is difficult to remove these particles from the surface of the X-ray absorbing film 8 (the X-ray absorbing pattern 5) by washing.